Why didn't MH370 emergency beacons work?

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Story highlights

One was designed to activate on impact, but satellites did not receive a distress signal

The lack of a distress signal boosts hopes of passengers' families

It is one of the most enduring mysteries of Malaysia Airlines Flight 370, and for the families, it's a reason for hope.

Why didn't Flight 370's emergency beacon work?

Why didn't the beacon send a distress signal to satellites overhead?

And the clincher: Why, if the beacon is designed to activate on impact, should we believe there was an impact? Could the plane have landed intact? Could the 239 passengers and crew still be alive?

The issue resonates with some family members looking for hope where little exists. Of the 26 questions families recently presented to the Malaysian government, 12 addressed the beacon.

Photos:The search for MH370

Photos:The search for MH370

Two years after Malaysia Airlines Flight 370 went missing, a relative of one of the passengers burns incense in Beijing on March 8, 2016. Flight 370 vanished on March 8, 2014, as it flew from Kuala Lumpur, Malaysia, to Beijing. There were 239 people on board.

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Photos:The search for MH370

On July 29, police carry a piece of debris on Reunion Island, a French territory in the Indian Ocean. A week later, authorities confirmed that the debris was from the missing flight.

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Photos:The search for MH370

Staff members with the Australian Transport Safety Bureau examine a piece of aircraft debris at their laboratory in Canberra, Australia, on July 20. The flap was found in June by residents on Pemba Island off the coast of Tanzania, and officials had said it was highly likely to have come from Flight 370. Experts at the Australian Transport Safety Bureau, which is heading up the search for the plane, confirmed that the part was indeed from the missing aircraft.

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Photos:The search for MH370

In late February, American tourist Blaine Gibson found a piece of plane debris off Mozambique, a discovery that renewed hope of solving the mystery of the missing flight. The piece measured 35 inches by 22 inches. A U.S. official said it was likely the wreckage came from a Boeing 777, which MH370 was.

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Photos:The search for MH370

Relatives of the flight's passengers console each other outside the Malaysia Airlines office in Subang, Malaysia, on February 12, 2015. Protesters had demanded that the airline withdraw the statement that all 239 people aboard the plane were dead.

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Photos:The search for MH370

A police officer watches a couple cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The couple's son was on the plane.

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Photos:The search for MH370

Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27, 2014. Data from communications between satellites and the missing flight was released the day before, more than two months after relatives of passengers said they requested it be made public.

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Photos:The search for MH370

Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14, 2014.

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Photos:The search for MH370

A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13, 2014.

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Photos:The search for MH370

The HMS Echo, a vessel with the British Roya; Navy, moves through the waters of the southern Indian Ocean on April 12, 2014.

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A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9, 2014.

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A relative of a missing passenger cries at a vigil in Beijing on April 8, 2014.

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Australian Defense Force divers scan the water for debris in the southern Indian Ocean on April 7, 2014.

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A towed pinger locator is readied to be deployed off the deck of the Australian vessel Ocean Shield on April 7, 2014.

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A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4, 2014.

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On March 30, 2014, a woman in Kuala Lumpur prepares for an event in honor of those aboard Flight 370.

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The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, 2014, after other relatives left en masse to protest the Malaysian government's response to their questions.

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A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27, 2014.

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Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27, 2014.

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People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27, 2014.

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Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24, 2014. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived."

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Grieving relatives of missing passengers leave a hotel in Beijing on March 24, 2014.

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A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22, 2014.

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A Chinese satellite captured this image, released on March 22, 2014, of a floating object in the Indian Ocean, according to China's State Administration of Science. It was a possible lead in the search for the missing plane. Surveillance planes were looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia.

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Satellite imagery provided by the Australian Maritime Safety Authority on March 20, 2014, showed debris in the southern Indian Ocean that could have been from Flight 370. The announcement by Australian officials raised hopes of a breakthrough in the frustrating search.

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Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight.

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A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19, 2014.

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On March 18, 2014, a relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet.

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U.S. Navy crew members assist in search-and-rescue operations in the Indian Ocean on March 16, 2014.

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Members of the Chinese navy continue search operations on March 13, 2014. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, search efforts expanded west into the Indian Ocean.

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A Vietnamese military official looks out an aircraft window during search operations March 13, 2014.

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Malaysian air force members look for debris near Kuala Lumpur on March 13, 2014.

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Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12, 2014.

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A member of the Vietnamese air force checks a map while searching for the missing plane on March 11, 2014.

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Photos:The search for MH370

A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported on March 8, 2014. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10, 2014.

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Photos:The search for MH370

A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews on March 9, 2014, before returning to search for the missing plane in the Gulf of Thailand.

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Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9, 2014.

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Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9, 2014.

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The rescue vessel sets out from Sanya in the South China Sea on March 9, 2014.

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Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8, 2014.

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A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8, 2014.

Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference at a hotel in Sepang on March 8, 2014. "We deeply regret that we have lost all contacts" with the jet, he said.

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Adding to the mystery: Hijackers or renegade pilots cannot disable some of the emergency beacons, namely, the ones attached to the plane's airframe. They are powered by batteries and inaccessible to the crew. So by all accounts, the attached beacon on Flight 370 should have activated if the plane crashed.

But experts consulted by CNN say there are numerous reasons why a beacon could fail in an ocean crash.

The beacon itself could be damaged by the impact, or its antenna could be sheared from the fuselage, rendering it inoperable.

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And there's one other possibility considered even more likely by some: The crash impact may have actually activated the beacon, but the damaged plane sank in less than 50 seconds, the time necessary for it to transmit its first emergency signal. The beacons do not work underwater.

On Friday, 49 days after Flight 370 disappeared, no one can say with certainty what happened with the plane's beacons.

That has left some family members with a faint glimmer of hope, but others believing that the beacon system just failed.

What are beacons?

Beacons -- more formally called emergency locator transmitters, or ELTs -- are devices that transmit an electronic distress signal in the event of a crash.

Unlike the so-called "pingers," which help investigators zero in on a plane's black boxes, ELTs are intended to help rescuers locate the plane itself. And, unlike the pingers, they do not operate when submerged under water, owing to the natural laws that govern radio waves.

Malaysia Flight 370 had four of them, Malaysian officials told CNN.

Two of the ELTs were stored with the airplane's life raft, to be activated by hand or by contact with the water, if the life boats were deployed.

The third ELT was stowed in the cabin.

But the ELT of greatest interest is the remaining "fixed" ELT, mounted to the aircraft frame. The fixed ELT -- a Honeywell RESCU 406 AFN -- was positioned near the rear door and connected to an antenna on top of the aircraft. It could be activated, either manually by a pilot in the cockpit, or automatically upon impact, by an inertial "G-switch."

The RESCU 406 AFN was designed "to provide emergency transmission for aircraft flying over land," according to Honeywell's published specifications.

"They are not mandated or designed to work under water," a Honeywell spokesman told CNN.

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But experts say any impact -- whether on land or at sea -- likely would have activated the transmitter.

Once activated, the device simultaneously transmits "bursts" -- short, digitally coded signals -- on three frequencies. Two of the frequencies -- 243 MHz and 121.5 MHz -- are VHF frequencies and can help search planes hone in on a target. The third frequency is 406 MHz.

That's where satellites come in.

Help from above

In 1979, the United States, Canada, France and the former Soviet Union teamed up to provide a global, satellite-based system to detect emergency beacons activated by planes, ships and backcountry hikers and to distribute those alerts to rescuers.

Known as the International Cospas-Sarsat Programme, the enterprise claimed it made its first rescue in September 1982, saving three people involved in the crash of a light aircraft in Canada.

"Cospas-Sarsat has done an enormous amount of good in the world, but almost nobody has ever heard of us," said Steven Lett, an American diplomat and head of the Cospas-Sarsat Secretariat in Montreal.

Cospas-Sarsat relies on six low-altitude, Earth-orbiting satellites and six high-altitude geostationary satellites, each with advantages and disadvantages.

The six low-altitude satellites, whose main function is to provide meteorological information, orbit the poles and give complete but non-continuous coverage of the Earth's surface. Because they can only view a portion of the Earth at any given time, the satellite may need to store geographic information from an emergency beacon and rebroadcast it when it comes within view of a ground facility.

The six geostationary satellites, parked in spots more than 22,000 miles above the equator, cover most of the Earth's surface but cannot determine the location of the beacon unless the location is encoded in the signal.

All of the satellites listen for a beacon's 406 MHz signals and together can identify a beacon's location to within approximately 3 kilometers, or just under 2 miles.

If Flight 370's ELT had transmitted a 406 MHz signal, it "almost certainly would have been picked up by one of the geostationary satellites," Lett said. Two satellites, India's Insat-3A and Russia's Electro-L1, are both parked over the Indian Ocean. It perhaps would have also been picked up by an orbiting low-altitude satellite.

Australia, Singapore, Indonesia and China all have antennas that monitor the satellites' emergency transmitter. Some or all of them likely would have received the distress call.

But authorities say no satellite signals were sent. No rescue was launched.

Other rescues

Cospas-Sarsat said about five people are rescued every day with the assistance of the satellite system.

But the disappearance of large commercial jetliners is very rare, and consequently, so is the discovery of them.

Cospas-Sarsat said it was instrumental in finding a Varig Airlines B-737 that wandered off course and crashed in the Brazilian jungle in September 1989. And when a Turkish Airlines B-737 dropped off radar near Schiphol Airport in the Netherlands in 2009, an ELT alert was the first confirmation to controllers that the aircraft had crashed.

The organization also said it was the primary or sole source of location information in about 25 other cases involving aircraft with 10 or more passengers.

Somewhere in the Indian Ocean

What can explain the lack of a signal in the Flight 370 case?

Assuming that the device was working correctly, the crash could have broken the antenna or cut the connection with the ELT, rendering it useless.

Another possibility, experts say, is that the aircraft could have sunk before the ELT began transmitting. It takes 50 seconds for the ELT to establish the necessary connection. It only takes one half-second data "burst" to indicate there is an emergency. But it can take a half-dozen bursts -- at the rate of one every 50 seconds -- to provide information that will allow Cospas-Sarsat to triangulate the beacon's position.

"In this case, there wasn't even one burst, according to the reports that we received," Lett said.

If the plane crashed in the southern Indian Ocean, as Malaysia Airline officials believe, the lack of a distress call could indicate that the plane plunged into the water, or sank quickly, because once underwater, the beacon is ineffective.

Likewise, the water-triggered ELTs in the life rafts would be ineffective if they became submerged, according to published Honeywell manuals for the devices.

Cospas-Sarsat also notes that beacons must have a relatively unobstructed view of the sky to work properly.

"A submerged beacon, or one with its antenna blocked by the body of an aircraft or vessel, is unlikely to be received by the satellites," the organization said.

Said Honeywell Aerospace spokesman Steven Brecken: "Until the recorders are recovered, we don't want to speculate what could or could not have happened. We ask the same thing you do, why didn't the ELT operate? We don't have the answer."

Family questions

In a recent letter to Malaysian authorities, a family group showered them with questions about the ELTs.

How many ELTs are on the plane, they asked, adding that they had gotten conflicting numbers.

Did Malaysia Airlines conduct maintenance checks? When was the latest check for MH370's ELT?

They asked to see the results of those checks.

Was the 406 MHz beacon certified? Was it possible to break the ELT in a crash? Where exactly was it located?

Would the ELT signal be weakened if it was surrounded by metal? Was the cable and blade antenna 9G certified? How much impact is needed to activate the ELT? Had the crew been trained in the use of ELTs? Can a beacon unlock "and bounce [float] to the surface of the water?"

Many of the questions remain unanswered.

But Cospas-Sarsat officials said that previous accidents have exposed shortcomings with the system and that they are working to improve it.

Among other things, they are testing a new constellation of mid-altitude satellites that can better determine the location of an ELT.

And government and industry officials are working on a new generation of ELTs that can monitor a plane conditions, identify problems and send a distress signal before the plane ever reaches the ground.